The invention is of particular interest for pixellated capacitive sensing systems, which can be used to determine material properties.
This type of pixellated capacitive sensing system can for example consist of an electronic circuit capable of measuring extremely small capacitances, originating from a chemical substance which changes its dielectric constant under the influence of a material to be detected. To obtain as much information as possible the sensor is not one big component, but an array of “pixels” (like image sensors). The pixellation enables measurement of gradients, and detection of material boundaries and imperfections.
An example is measuring the amount of CO2 in ambient air.
There are a number of different ways to measure capacitance. The capacitor under measurement can be driven with either a voltage or a current excitation signal. The output signal can be the current or voltage that flows as a result of the excitation, but it can also be a ratio or difference signal.
Examples of possible combinations of excitation signal and readout signal are:
(i) AC voltage drive, measurement of the AC current through capacitor;
(ii) AC current drive, measurement of the AC voltage across capacitor;
(iii) AC voltage drive, measurement of the DC current through switched capacitor;
(iv) AC current drive, measurement of the DC voltage across switched capacitor;
(v) AC voltage drive, measurement of the capacitance ratio using a bridge circuit;
(vi) AC current drive, measurement of the capacitance ratio using a dual slope converter;
(vii) AC voltage drive, measurement of the capacitance difference using a switched difference integrator;
(viii) AC current drive, measurement of the capacitance difference;
(ix) Determination of an oscillation frequency dependent on the capacitance, for example using an oscillator.
Known pixelated sensors (also known as array impedance sensors) generally use AC voltage driving with measurement of the AC current through a capacitor (method (i) above). The change of the capacitance is interpreted to determine substance properties. A known biosensor makes use of AC voltage drive with measurement of the DC current through a switched capacitor (method (iii) above).
There is a need for a measurement approach which is accurate and can be implemented with simple circuitry.